The Lewis of Shadowlands

I really enjoyed the Shadowlands movie, but I am now curious how well the character of Lewis was portrayed.  Possibly because of the stories I have heard such as filling his study with smoke or from the stories he wrote, I have always imagined Lewis having an almost childlike mischievous personality.  His stories are deep and intelligent yet speak so well to children, I find it hard to believe these came from a stern man, such as the one portrayed in shadowlands.  Rarely do you find a stern person with such an imaginative mind.  I think the movie missed on portraying the character of Lewis.